libsnapshot: SnapshotMergeStats::Start/Finish

Change API for SnapshotMergeStats so that it is easier
for update_engine to use.

Test: apply OTA and look at stats
Bug: 147696014
Bug: 138817833
Change-Id: Ie4036ac7382102c00f0761f443d78e00b9e585d5
Merged-In: Ie4036ac7382102c00f0761f443d78e00b9e585d5
4 files changed